This invention is directed to electrons guns and, in particular, to off-axis electron guns.
The most common type of electron gun consists of a point source that is surrounded by a focus electrode to focus and drive the electrons in a forward direction through an anode. One example of such a source is disclosed in U.S. Pat. No. 3,694,687, which issued on Sept. 26, 1972. This source would usually be located on the beam axis of the device that uses the electron beam. However, for accelerators such as the one disclosed in U.S. Pat. No. 4,006,422, that issued on Feb. 1, 1977, to Atomic Energy of Canada Limited, in which the beam makes a double pass through the accelerator structure to provide an efficiently accelerated beam within a compact structure, the electron source cannot be located on the beam axis. In this case, a source, with an electrostatic or a magnetic deflector, such as disclosed in U.S. Pat. No. 2,839,706, which issued on June 17, 1958 to G. E. Anderson et al., would be used.
However, in commercial low cost accelerator systems, such as those used in the medical field, it is generally acknowledged that the voltage pulses delivered to the electron gun may vary by up to 5%, which will affect the deflection of the beam by the injection system. In addition, such deflectors do not generally come within the size restrictions imposed on such systems due to the large deflector angle required.